This invention relates to a method and apparatus for improving the capability and reliability of systems for detecting the presence, speed, and direction of movement of a metallic object and, in particular, a railway vehicle by improving the ability to detect a vehicle wheel both when moving and at zero speed.
Transducers are used to detect the wheels of a moving railway vehicle to activate automatic gates at railroad crossings or determine the exact position of a railway vehicle for the transfer of a load from a railway car to a collection site. One such detector is disclosed in U.S. Pat. No. 5,395,078 to Gellender. One problem with the wheel transducer described in the Gellender patent has been that the transducer cannot detect a wheel stopped at the magnetic center of the two coils. A further problem with prior art transducers has been the need to calibrate periodically due to baseline voltage drift caused by temperature variations, vibration, and debris, which effect the performance of the transducer components. A system tolerant to this drift is particularly important to a zero speed transducers.